H.323 is a standard promulgated by the International Telecommunications Union (ITU) for multimedia communications over local-area networks (LANs) utilizing Internet Protocol (IP) or another packet-switched medium. The H.323 standard is attractive, for one, because it is a flexible standard appearing in a field dominated by proprietary designs that offer little hope for interoperability between different vendor's equipment. Thus, H.323 offers the hope of a world where different vendor's equipment and different carrier's networks can and will communicate seamlessly. The H.323 standard is also attractive because it allows an administrator some measure of control over the amount of voice, video, and other multimedia traffic traversing a packet-switched network that has no other quality-of-service guarantees.
An H.323 call requires that several connections be set up between the calling endpoints. The first of these is a TCP/IP connection for the communication of basic call functionality. ITU-T recommendation H.225 describes the call signaling functionality that is provided on this channel (a variation on Q.931 signaling).
The second connection is a control channel that orchestrates multimedia data communications between the endpoints. This second connection set up between endpoints is also a TCP/IP connection (note that with H.323 version 2, a single TCP/IP connection can in some cases be used for both the first and second connections). ITU-T recommendation H.245 describes the control protocol to be used on this channel. A number of different services may be provided over an H.245 call control channel, including master/slave endpoint determination, audio/visual and data capabilities exchange, and the management of logical channels used to transport audio/visual and data information.
In particular, the audio/visual and data capabilities exchange is intended to ensure that the only multimedia signals that are transmitted are those that can be received and understood by a receiving endpoint. To implement this, each endpoint transmits a capability set to the other, indicating what types and combinations of information streams it can accept. For example, H.323 allows endpoints to implement one or more audio codecs, including ITU audio codec specifications G.711 (required), G.722, G.723.1, G.728, and G.729, some of these having multiple possibilities for bit rate and other settings. An endpoint can, e.g., identify G.711, G.722, and G.729 in its capability set, signaling its peer endpoint that it may choose to open a logical channel for any one of these three identified audio codecs, but not for any other.
Because each H.323 transmitting endpoint can open a logical channel corresponding to any one of its peer's advertised capabilities, it is possible that the endpoints will select different transmit capabilities. This results in the creation of asymmetrical logical channels, e.g., a G.711 audio channel in one direction and a G.729 audio channel in another.
Many systems can simultaneously operate one codec for transmitting, and another for receiving, thus allowing them to function with asymmetrical logical channels. But in some terminal configurations, it may be advantageous or necessary to constrain an endpoint to only symmetrical codec selection. If asymmetrical logical channels are set up with such an endpoint, its user may hear incoherent audio or nothing at all.